A pedestrian suffered multiple injuries when he was hit by a white Ford Transit Van.
The 42-year-old man from Hemel Hempstead is believed to have parked his silver Nissan pick-up truck in a lay-by near Phasels Wood, on the A41, on Friday before moving into the road where he was hit by the van.
He was taken to Hemel Hempstead hospital and is in a stable but serious condition.
The driver of the van, a 51-year-old man from Chesham, and his passenger were uninjured in the incident, which happened at 1.15pm.
The northbound A41 carriageway was closed between junction 20 of the M25 and the Hemel Hempstead turning for several hours.
PC Duncan Montague said: "We are looking to speak to anyone who may have seen the incident or who saw the silver Nissan parked in the lay-by prior to the incident."
Anyone with information can telephone the western area road policing unit on 01923 472380 or 0845 3300222.
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